Radiance-general Digest, Vol 94, Issue 1

Gidday Mostapha

Personally, I like the cumulative approach that John has demonstrated - climate based analysis is always going to be much more informative. It would avoid the issues raised by Andy in relation to the UK 'Right to Light'/mildewed courtyard issue.

I do want to return to your calculation though. You state you have a mental picture of tracing the light from the sensor to the ceiling etc... If the metric is for houses, this seems quite an odd approach. You might do well to read about overall / average DF - perhaps starting with Joe Lynes 1979 paper: DOI: 10.1177/1477153579011002010 : A sequence for daylighting design : Lighting Research and Technology 1979 11: 10

IMHO the likely general cheerfulness (brightness) of a home, however imprecisely tested, is of more interest than the illuminance on a desk or table?



Victoria University of Wellington School of Architecture
Michael Donn
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Today's Topics:

   1. Re: Daylighting metric for outdoor spaces (Mostapha Sadeghipour)


Message: 1
Date: Wed, 30 Nov 2011 19:27:27 -0600
From: Mostapha Sadeghipour <[email protected]>
To: Radiance general discussion <[email protected]>
Subject: Re: [Radiance-general] Daylighting metric for outdoor spaces
Content-Type: text/plain; charset="iso-8859-1"

Hi Andy,

Thank you for clarification and explanation. It was so informative.

I agree that using hours of sunshine (sunlight hours) in this way totally
make sense however in the code I mentioned it was a metric for building
envelopes (e.g. two hours of sunlight hours in 21st December is needed for
residential buildings).

As you said the weakness of metrics like VSC or Daylight Factor
(SC+ERC+IRC) is not being climate-based and orientation sensitive but maybe
this is the best possible way for now.


On Wed, Nov 30, 2011 at 12:18 PM, Andy McNeil <[email protected]> wrote:

Hi Mostapha,

BRE has a metric - Vertical Skylight Component - for what you describe.
It is intended as a planning tool to insure that housing blocks are not
located to close together. It is also used in rights to light litigation
in the UK. It's independent of climate though, it's mainly to ensure that
an appropriate percentage of daylight is made available at the window (i.e.
not too many tall buildings nearby). I'm not saying it's a great metric,
but it's commonly used for what you want to do.

VSC is commonly coupled with the hours of sunshine in a courtyard metric
you mentioned in an earlier email. I just want to point out that this is
not a daylight metric but a planning tool to ensure that courtyards are not
more damp and dank than an open park in the same climate. For example, if
you have a week of damp overcast weather both your open space and your
courtyard will be damp, but then when you have a day of sunshine your park
will dry up, but your courtyard may not if there isn't sufficient sun
penetration. A courtyard that stays damp gets even worse in the next week
of wet weather. The metric is intended to prevent mold, mildew and
perpetually damp courtyards. So in reality this metric is more applicable
to climates like London and Seattle than Phoenix or Dubai.


On Nov 30, 2011, at 9:31 AM, Mostapha Sadeghipour wrote:

Hi John, Michael, et al.


Sorry if it wasn't clear enough. I think you get the concept. Yes! I
wonder if there is a number to determine the outdoor illuminance to provide
enough illuminance level inside. What you are saying is true and the
effective parameters are much more than only VLT and ceiling reflectance
but if I want to consider all of them I should modify the geometry and run
the lighting simulation.

The way I did the ray-tracing in my mind was the simplest possible
backward ray-tracing. I started from the sensor inside, then traced only
one ray upward and bounced it from the ceiling to the glazing. So
illuminance level inside the space is equal to illuminance level outside
the glazing (on the envelope of the building that I can simulate) multiply
by VLT of the glazing multiply by the reflectance of the ceiling. In this
way I can make a target for the illuminance level on the envelopes rather
than indoor.

The question could be more general though and thinking about the relation
between outdoor illuminance level and then the external envelopes.


Thank you for the great link! This is exactly what I'm talking about. I
actually ran the accumulative annual study. That's true that cumulative
result shows you get less illuminace in more dense areas but what are the
cutting levels? Do we have the concept of over-daylit for outdoors?

For example if I calculate the availability of useful daylight illuminance
based on 100 lux and 2500 lux for working hours and calculate the result
based on the percentage of hours a large portion of the well-daylit outdoor
spaces will be considered as over-daylit because they receive more than
2500 lux, and then the more dark outdoor spaces located in Seattle or
London (hi Rob!) will be assumed as well-daylit since they will be in the
range. We already knew this is not true and the spaces next to an outdoor
space with 150 lux horizontal illuminance level cannot be well-daylit.

Maybe we can say there should be no upper-limit for outdoor illuminace
level since we can always mitigate the light level by building envelope
design, but what is the lower limit then?


PS.1: I liked the concept of "warm shade" so much. So interesting to think
about.... Can you send me a link to the paper or any other resources?

On Wed, Nov 30, 2011 at 10:26 AM, John Mardaljevic <[email protected]> wrote:

Hi Mostapha,

> For an urban design study I wanted to avoid measuring light levels
inside the buildings as far as possible.

How about using cumulative values, say annual or maybe monthly? You
could also look at just the hours of occupancy. Some examples here:


The image for London clearly shows the effect of tall buildings reducing
the ground level cumulative illuminance (actually, irradiance in the legend
but just x 100 to estimate klux-hrs / yr).


Reader in Daylight Modelling
Institute of Energy and Sustainable Development
De Montfort University, The Gateway, Leicester, LE1 9BH, UK
Tel: +44 (0) 116 257 7972

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End of Radiance-general Digest, Vol 94, Issue 1